1. Field of the Invention
The present invention relates to the welding of pipe systems used in the chemical industry. More specifically, the present invention relates to an improved welded flange connection and to methods and apparatus for producing such a connection by thermowelding plastic pipes or plastic-lined pipes.
2. Description of the Background
Plastic pipe systems and plastic-lined pipe systems are used in industrial plants to transport corrosive chemicals. These pipe systems are constructed of a plurality of plastic pipes or plastic-lined pipes fitted together using flange connections. The typical chemical plant will employ thousands of flange connections to produce several miles of interconnected plastic pipe or plastic-lined pipe. Many of the chemicals carried by these pipe systems are hazardous. Thus, any release of chemicals, even a small one, due to a failure in these pipe systems, can cause significant damage to the environment and to human life. Accordingly, the integrity of these thousands of welded flange connections is critical.
Because of environmental concerns, chemical plants must monitor these flange connections on a regular basis. Even the smallest leak must be reported to environmental authorities. Monitoring is often accomplished by sniffing each flange connection at a monthly cost which may range up to as much as $15.00 per connection. Thus, in the typical plant, monitoring costs often exceed several hundred thousand dollars per year. However, even with constant monitoring, there are no guarantees that leaks will not develop. Thus, the industry has long sought improved connections and methods of producing those connections.
The most commonly used method for welding plastic pipe or plastic-lined pipe is butt-welding. With plastic pipe systems, adjacent flanged pipe sections are merely welded by conventional thermowelding of the abutted ends of pipe sections. With plastic-lined systems, the plastic liner is first butt welded. After the liner is welded, a steel coupling is swaged or welded over the thermowelded connection of the plastic liner. While providing a welded seal, these systems do not permit the thermowelded connection to be hydrotested at the final operating pressure prior to placement of the steel coupling over the connector. Further, the welds in most of these systems are permanent, i.e., the thermowelded connection cannot be reversed and separated. Therefore, if a section of the plastic pipe or lining should fail, or if the welded connection itself should fail or need to be replaced, the connection will have to be cut out and re-welded from each end. These conventional thermowelding methods and connections have been described in a number of patents.
For example, the Wallace patent (U.S. Pat. No. 2,992,838) discloses a pipe joint having a heated sealing means. In Wallace, a pipe joint is sealed by first bolting together two sections of pipe to form a mechanical seal. The pipe sections are secondarily sealed using a small strip of thermoplastic material which, when heated, is fused between the sealed adjacent faces of two small fins extending from the outer surface of each pipe section. Thus a seal is formed between the two fins. While a thermoplastic seal is formed between the pipe sections by way of these small fins, the seal created is a weak seal which would most likely be incapable of withstanding high pressure. Further, the seal, itself, is directly exposed to the pipe interior and would be subject to attack by any corrosive materials therein. Thus, the seal disclosed in Wallace is not suitable for joining pipe transporting hazardous or corrosive materials under high pressure.
The Hunter patent (U.S. Pat. No. 3,968,552) discloses a method and apparatus for forming a plastic-lined junction in plastic-lined pipe. In Hunter, a groove is first forming in the outer surface of each pipe at a predetermined distance from the end of each pipe section to be joined. A compression ring is then fitted into each groove. A flange is then mated with each compression ring. A plastic end-face liner is then inserted through the open end of each pipe section and bonded to the plastic lining of the pipe. The abutting faces of the flanges on each pipe section are then bolted together and, for added rigidity, sealed with an epoxy or other suitable material. The Hunter connection, while suitable for sealing plastic-lined pipe, does not allow for rapid repair. A portion of the pipe must be cut out and removed to repair the connection.
These and other prior art systems suffer from the aforementioned disadvantages. Thus, the chemical industry has continued to seek improved connections and methods for producing those connections for use with plastic pipe systems and, particularly, plastic-lined pipe systems. Accordingly, there has been a long felt need for a method of welding plastic or plastic-lined pipe to produce a solid weld, shielded from the corrosive contents of the pipe system, yet accessible for easy testing. Further, such a weld should be reversible to facilitate repair and replacement where necessary. Finally, such welds and methods of producing them must be economical and efficient to use. The present invention solves those needs.